Piston including a composite layer applied to a metal substrate

ABSTRACT

A piston for a heavy duty diesel engine including a composite layer forming at least a portion of a combustion surface is provided. The composite layer has a thickness greater than 500 microns and includes a mixture of components typically used to form brake pads, such as a thermoset resin, an insulating component, strengthening fibers, and an impact toughening additive. According to one example, the thermoset resin is a phenolic resin, the insulating component is a ceramic, the strengthening fibers are graphite, and the impact toughening additive is an aramid pulp of fibrillated chopped synthetic fibers. The composite layer also has a thermal conductivity of 0.8 to 5 W/m·K. The body portion of the piston can include an undercut scroll thread to improve mechanical locking of the composite layer. The piston can also include a ceramic insert between the body portion and the composite layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This U.S. divisional patent application claims the benefit of U.S.utility patent application Ser. No. 15/389,862, filed Dec. 23, 2016which claims the benefit of U.S. provisional patent application No.62/271,425, filed Dec. 28, 2015, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates generally to pistons for internal combustionengines, such as insulated heavy duty pistons for diesel engines, andmethods of manufacturing the same.

2. Related Art

Modern heavy duty diesel engines are being pushed by legislation andcustomer demands towards increased thermal brake efficiency. The targetthermal brake efficiency is currently 46%, but is expected to be up to60% by the year 2025. Thus, heavy duty pistons with reduced heat flowthrough the crown, and thus reduced overall energy loss, are desired.Reducing heat flow through the crown allows more energy to be retainedin the hotter exhaust gases, and some of this energy can be recoveredand converted to useful work by turbo-compounding. For example, certainengine manufacturers desire a mechanism to reduce heat flow through thecrown by 50%.

One way to insulate and reduce heat flow through the piston crown is byapplying a ceramic coating, for example by thermal spraying. However,such ceramic coatings have a thickness limit, typically 500 microns.Although a greater thickness would provide better insulation, a thickercoating has the risk of spalling and delamination due to the differencesin thermal properties between the metal substrate and the coating.Ceramic coatings formed to a thickness greater than 500 microns riskdelamination and spalling, even if a metal bond layer is applied beforethe ceramic coating. This poses a challenge because simulations haveshown that reducing heat flow by 50% may not be possible with a ceramiccoating if the thickness is less than 500 microns, even though thethermal conductivity of the ceramic coating is low, typically 0.2 to 1.0W/m·K. Thus, a thicker and/or more robust coating of low thermalconductivity material may be required. The coating material must alsoadhere well to the top surface of the metal substrate and be able towithstand combustion temperatures of about 800° C. and peak pressures ofabout 250 bar.

SUMMARY OF THE INVENTION

One aspect of the invention provides a piston for use in an internalcombustion engine, such as a heavy duty piston for a diesel engine. Thepiston includes a body portion formed of metal, and a composite layerapplied to the body portion. The composite layer forms at least aportion of a combustion surface of the piston and has a thickness ofgreater than 500 microns. The composite layer includes a thermosetresin, an insulating component, strengthening fibers, and an impacttoughening additive.

Another aspect of the invention provides a method of manufacturing thepiston. The method includes applying a composite layer to a body portionformed of metal. The composite layer forms at least a portion of acombustion surface, the composite layer has a thickness of greater than500 microns, and the composite layer includes a thermoset resin, aninsulating component, strengthening fibers, and an impact tougheningadditive.

The composite layer provides improved insulation of the piston duringuse in the internal combustion engine, compared to a ceramic coating, byreducing heat flow through the crown. Thus, the composite layer allowsmore energy to be retained in the hotter exhaust gases, which can beconverted to useful work and lead to improved thermal brake efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated,as the same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings wherein:

FIG. 1 is a perspective view of a diesel engine piston including acomposite layer forming a portion of a combustion surface according toan example embodiment of the invention;

FIG. 2 is a sectional view a comparative diesel engine piston includinga ceramic coating applied to a crown of the piston instead of thecomposite layer;

FIG. 3 is a top view of a puck used to simulate a metal body portion ofthe piston shown in FIG. 1 before applying the composite layer andshowing an undercut scroll thread formed along an uppermost surface ofthe metal body portion;

FIG. 4 is an enlarged sectional view of the undercut scroll thread ofFIG. 3 after applying the composite layer;

FIG. 5A is another enlarged sectional view of the undercut scroll threadof FIG. 3 after applying the composite layer;

FIG. 5B is an enlarged sectional view of the undercut scroll thread ofFIG. 3 after applying the composite layer, wherein a ceramic insert islocated between the composite layer and the metal body portion;

FIGS. 6-13 illustrate steps of a method of manufacturing the piston ofFIG. 1 according to an example embodiment; and

FIG. 14 is a sectional view of the finished piston including thecomposite layer according to an example embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

One aspect of the invention provides a piston 20 for use in an internalcombustion engine, such as a piston 20 for a heavy duty diesel engine.The piston 20 includes a composite layer 22 molded to an uppermostsurface 34 of a body portion 26, also referred to as a substrate, whichis formed of metal. The composite layer 22 is formed of ingredientstypically used to manufacture automotive brake pads and has a thicknessgreater than 500 microns. Thus, the composite layer is 22 is expected toreduce heat flow through a crown 32 of the piston 20 by at least 50%.The composite layer 22 is also expected to maintain good adhesion andwithstand combustion temperatures of about 800° C., and peak pressuresof about 250 bar. The piston 20 including the composite layer 22according to one example embodiment is shown in FIG. 1. Although theexample piston 20 is designed for use in a heavy duty diesel engine, thecomposite layer 22 could be used in other types of pistons.

The composite layer 22 formed of the brake materials can be used inplace of a ceramic coating, such as the ceramic coating 21 applied tothe comparative piston 20′ shown in FIG. 2. In the comparative piston20′ of FIG. 2, the ceramic coating 21 is applied to the uppermostsurface 34 and a ring land 38 to reduce heat loss to the combustionchamber and thus increase efficiency of the engine. Thus, the ceramiccoating 21 forms a combustion surface which is directly exposed to theextreme conditions of the combustion chamber. The piston 20 of thepresent invention can have a design similar to the design of the piston20′ shown in FIG. 2.

FIG. 3 is a top view of a puck used to simulate a metal body portion 26of the piston 20 shown in FIG. 1 before applying the composite layer 22.FIGS. 4 and 5 are enlarged sectional views of an undercut scroll thread56 of the piston 20 of FIG. 3 after applying the composite layer 22.FIGS. 6-13 illustrate steps of a method of manufacturing the piston 10according to an example embodiment; and FIG. 14 is a sectional view ofthe finished piston 20 including the composite layer 22 according to anexample embodiment.

The piston 20 of FIG. 14 includes the metal body portion 26 extendingaround a center axis A and longitudinally along the center axis A froman upper end 28 to a lower end 30. The body portion 26 also includes acrown 32 extending circumferentially about the center axis A from theupper end 28 toward the lower end 30. In the embodiment of FIG. 2, thecrown 32 is joined to the remainder of the body portion 26, in this caseby welding. The body portion 26 of the piston 20 can be formed ofaluminum, steel, or another metal material.

The crown 32 of the piston 20 of FIG. 14 includes the uppermost surface34 at the upper end 28 which is exposed to hot gasses, and thus hightemperatures and pressures, during use of the piston 20 in the internalcombustion engine. The uppermost surface 34 defines a combustion bowlextending inwardly and downwardly from a planar outer rim. The crown 32of the piston 20 also defines at least one ring groove 36 extendingcircumferentially about the center axis A for receiving at least onering (not shown). Typically the piston 20 includes two or three ringgrooves 36. Ring lands 38 are disposed adjacent each ring groove 36 andspace the ring grooves 36 from one another. The piston 20 also includesa cooling gallery 24 extending circumferentially around the center axisA between the crown 32 and the remainder of the body portion 26. In thisembodiment, the crown 32 includes an upper rib 42 spaced from the centeraxis A, the adjacent section of the body portion 26 includes a lower rib44 spaced from the center axis A, and the upper rib 42 is welded to thelower rib 44 to form the cooling gallery 24. In this case, the ribs 42,44 are friction welded together, but the ribs 42, 44 may be joined usingother methods. The cooling gallery 24 can contain a cooling fluid todissipate heat away from the hot crown 32 during use of the piston 20 inthe internal combustion engine. In addition, cooling fluid or oil can besprayed into the cooling gallery 24 or along an interior surface of thecrown 32 to reduce the temperature of the crown 24 during use in theinternal combustion engine.

The body portion 26 of the example piston 20 of FIG. 14 further includesa pair of pin bosses 46 spaced from one another about the center axis Aand depending from the crown 32 to the lower end 30. Each pin boss 46defines a pin bore 48 for receiving a wrist pin which can be used toconnect the piston 20′ to a connecting rod. The body portion 26 alsoincludes a pair of skirt sections 54 spacing the pin bosses 46 from oneanother about the center axis A and depending from the crown 32 to thelower end 30.

According to the present invention, however, the ceramic coating 21,which is applied to the piston 20′ of FIG. 2 is not used, and insteadthe composite layer 22 is molded to the uppermost surface 34 of thecrown 32. The composite layer 22 is molded to the uppermost surface 34and forms the entire combustion surface of the piston 20, or a portionof the combustion surface of the piston 20. The composite layer 22 couldalso be applied to at least one of the ring lands 38. According to oneexample embodiment, the composite layer 22 is molded along only aportion of the uppermost surface 34 of the crown 32, for example on theouter rim, as shown in FIG. 14.

The piston 20 of FIG. 14 also includes the undercut scroll thread 56formed in the uppermost surface 34 to improve mechanical locking of thecomposite layer 22 to the uppermost surface 34. The undercut scrollthread 56 is typically formed in the areas where the composite layer 22is applied, which can include the combustion bowl and/or the areasurrounding the combustion bowl. FIG. 3 is an aluminum puck simulatingthe metal body portion 26 including the undercut scroll thread 56 formedin the uppermost surface 34. The puck of FIG. 3 has a diameter of 104millimeters and includes a pocket to simulate the combustion bowl, andthe undercut scroll thread 56 is also formed in the pocket. FIGS. 4 and5 are enlarged sectional views of the undercut scroll thread 56 of FIG.3 after applying the composite layer 22.

The composite layer 22 is formed of ingredients typically used to formautomotive brake pads. The ingredients are blended to form a compositemixture which can be molded directly to the uppermost surface 34 of thebody portion 26. The composite layer 22 has a thickness of greater than500 microns, for example 2 to 3 millimeters. It is expected that thecomposite layer 22 will withstand engine temperatures and pressures,since brake materials are typically molded at pressures of about 345 barwithout damage, and experience temperatures in excess of 600° C. underhard braking conditions when used in service pads of rotors.

The composite layer 22 of the example embodiment has a thermalconductivity of 0.8 to 5 W/m·K, for example about 1 W/m·K. However, theratio of ingredients can be adjusted to adjust the thermal properties ofthe composite layer 22. In the example embodiment, the composite layer22 includes a mixture of thermoset resin, insulating component,strengthening fibers, and impact toughening additive. Different types ofthermoset resin could be used to form the composite layer 22, but in theexample embodiment, the thermoset resin is a Novalac type phenolicresin. According to the example embodiment, the thermoset resin ispresent in an amount of 25 weight percent (wt. %) to 35 wt. %, based onthe total weight of the composite layer 22. Different types ofinsulating components could be used to form the composite layer 22, butin the example embodiment, the insulating component is a ceramic fiberor powder. According to the example embodiment, the insulating componentis present in an amount of 50 wt. % to 70 wt. %, based on the totalweight of the composite layer 22. Different types of strengtheningfibers could also be used to form the composite layer 22, but in theexample embodiment, the strengthening fibers are formed of graphite.According to the example embodiment, the strengthening fiber is presentin an amount of 1 wt. % to 10 wt. %, based on the total weight of thecomposite layer 22. Different types of impact toughening additives couldalso be used to form the composite layer 22, but in the exampleembodiment, the impact toughening additive is fibrillated Kevlar®, whichis an aramid pulp of highly fibrillated chopped synthetic fibers.According to the example embodiment, the impact toughening additives ispresent in an amount of 1 wt. % to 10 wt. %, based on the total weightof the composite layer 22. For example, the composite layer 22 can beformed of a mixture including 10.4 grams Novalac phenolic resin, 17.9grams Superwool® 607® ceramic fibers, 0.9 grams of ¼ inch graphitefibers, and 0.6 grams crushed Kevlar®. The example mixture provides acomposite layer 22 having a thickness of 2 millimeters when applied tothe 104 millimeter diameter aluminum puck shown in FIG. 3. The compositelayer 22 formed of the example mixture also has a thermal conductivityof about 1 W/m·K.

To further improve the performance of the piston 20, a ceramic insert 58can be located between the uppermost surface 34 the metal body portion26 and the composite layer 22, as shown in FIG. 5B. The ceramic insert58 can be placed in a desired location or locations along the uppermostsurface 34 the metal body portion 26, for example the areas oppositefuel plumes where a flame front typically contacts the bowl region, orother locations subjected to the most aggressive conditions in thecombustion chamber. According to one embodiment, the ceramic insert 58is formed of an alumina ceramic.

Another aspect of the invention provides a method of manufacturing thepiston 20 with the composite layer 22 forming at least a portion of thecombustion surface. The method generally includes molding a compositemixture of ingredients typically used to form brake pads to theuppermost surface 34 of the piston body portion 26. FIGS. 6-13illustrate steps of an example method used to form the example piston 20of FIG. 1. However, other methods could be used.

The method begins by obtaining the composite mixture, or preparing themixture from raw ingredients. Only thermal and structural ingredientsare needed, as there is no need for friction modifiers or non-usefulfillers which are used in brake pad materials. In the exampleembodiment, the method includes obtaining the thermoset resin,insulating component, strengthening fibers, and impact tougheningadditive, as shown in FIG. 6, and then loading the ingredients into amixing chamber, as shown in FIG. 7. In the example embodiment, thethermoset resin is a Novalac type phenolic resin, the insulatingcomponent is a ceramic fiber or powder, the strengthening fibers areformed of graphite, and the impact toughening additive is fibrillatedKevlar®. For example, the mixture can include 10.4 grams Novalacphenolic resin, 17.9 grams Superwool® 607® ceramic fibers, 0.9 grams of¼ inch graphite fibers, and 0.6 grams crushed Kevlar®.

FIG. 8 shows a mixer containing the ingredients of the exampleembodiment. The mixer of FIG. 8 is a V-mixer, but the mixer couldalternatively be a plough share mixer or another type of mixer. FIG. 9shows the composite mixture after the mixing step. After obtaining orpreparing the composite mixture, the composite mixture is loaded into amold tool, as shown in FIG. 10. The mold tool includes a press that canapply up to 2.5 tons/square inch, or 5000 psi to a surface area. Themold tool also includes a means of heating, such as platens that canreach up to 250° C., which allows for a cure and flow cycle that causesthe composite mixture to flow, B-stage, and then cure.

The method further includes obtaining or providing the body portion 26of the piston 20, which is formed of metal, such as aluminum or steel.This step typically includes forming the undercut scroll thread 56 alongthe uppermost surface 34 of the body portion 26, as shown in FIG. 11, inpreparation for the molding step. The body portion 26 is then placed ina molding tool, along with the composite mixture, as shown in FIG. 12.

The method next includes molding the composite mixture to the uppermostsurface 34 of the body portion 26 to form the composite layer 22. Themolding step, specifically the flow and cure cycle, is modified fromthat of brake pad, block, or shoe material manufacturing in order toprevent problems that can arise from gas liberated during cure of theresin matrix material. For example, in brake pads, cracks along thecenter plane of the brake pad form as the cure gases force their escape.

The improved method of the present invention includes a B-stagingoperation, which is a very low degree of curing at 120 to 130° C. for 5to 60 minutes, for example about 15 minutes, to reduce the tendency forthe resin to rapidly emit gas while curing. During the B-staging step,the resin is still able to melt and flow along the uppermost surface 34and conform to the shape of the undercut scroll thread 56, but does notfully cure. In the example embodiment, the method includes heating themold tool to a temperature of 130° C. by the heated platen, loading thecomposite mixture and the piston body portion 26 into the mold tool, andthen compressing the mixture and body portion 26 together at 1ton/square inch. The B-staging step then includes holding the compressedmixture and body portion 26 at 130° C. for 15 minutes.

After the B-staging step, the method includes increasing the temperatureof the compressed composite mixture and body portion 26 for a period oftime so that the composite mixture cures and forms the composite layer22. In the example embodiment, the method includes increasing thetemperature for 10 to 60 minutes, for example 11 minutes, to reach atemperature of 180 to 250° C., for example about 200° C. in the moldtool. The method then includes holding the composite mixture and bodyportion 26 at the elevated temperature, for example 200° C. for anadditional 15 minutes, before removing the piston 20 from the mold tool.The temperature of the curing step of the present method is higher thanthat typically used to form brake pads, which is less than 180° C. Thus,the resin has a greater cure and better mechanical properties. As shownin FIG. 14, the finished piston 20 includes the composite layer 22securely molded to the uppermost surface 34 of the body portion 26.

Other methods can alternatively be used to form the piston 20 includingthe composite layer 22. For example, the method could be optimized toreduce the cycle time. As an alternative to loosely filling the moldtool with the composite mixture, the composite mixture could be firstmolded into a preform insert and then B-staged such that a flatdisc-shaped insert with a hole for the combustion bowl is formed. Theinsert is then dropped into a mold assembly, and the metal body portion26 is placed on top of the insert before curing. This alternative methodmay be better suited for a production environment.

To further improve the performance of the piston 20, the method canoptionally include disposing the ceramic insert 58 along the uppermostsurface 34 of the metal body portion 26 before molding the compositemixture to the ceramic insert 58 and the metal body portion 26. Forexample, the ceramic insert 58 can be formed of alumina ceramic andplaced in locations typically subjected to aggressive conditions in thecombustion chamber.

The composite layer 22 formed by the method of the present invention hasa thermal conductivity similar to thermal spray ceramic coatings, butcan be formed to a thickness of greater than 500 microns. Thus, thecomposite layer 22 is more effective at insulating the piston 20 duringused in the internal combustion engine. In addition, the compositemixture can be molded into very complex shapes without line-of-sightissues which oftentimes exist in plasma spray or high velocity oxygenfuel (HVOF) spray of ceramic coatings.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings and may be practicedotherwise than as specifically described while within the scope of theclaims.

The invention claimed is:
 1. A method of manufacturing a piston,comprising the steps of: applying a composite layer to a body portionformed of metal, the composite layer forming at least a portion of acombustion surface, and the composite layer including a thermoset resin,an insulating component, strengthening fibers, and an impact tougheningadditive, wherein the thermoset resin is a phenolic resin, theinsulating component is a ceramic, the strengthening fibers aregraphite, and the impact toughening additive is an aramid pulp offibrillated chopped synthetic fibers.
 2. The method of claim 1, whereinthe step of applying the composite layer to the body portion includesmolding a mixture to an uppermost surface of the body portion.
 3. Themethod of claim 2, wherein the molding step includes a B-stage, theB-stage includes applying heat and pressure to the mixture on the bodyportion, and the mixture does not fully cure during the B-stage.
 4. Themethod of claim 3, wherein the molding step further includes increasingthe temperature of the mixture while applying pressure to the mixtureafter the B-stage until the mixture fully cures.
 5. The method of claim1, wherein the step of applying the composite layer to the body portionincludes forming a preform insert of a mixture, and molding the preforminsert to an uppermost portion of the body portion.
 6. The method ofclaim 1, wherein the step of applying the composite layer to the bodyportion includes disposing an insert formed of ceramic on an uppermostsurface of the body portion and disposing the composite layer on theceramic insert.
 7. The method claim 1, wherein the composite layer has athermal conductivity of 0.8 to 5 W/m·K.
 8. The method of claim 1,further including the steps of: providing the body portion formed ofaluminum or steel; forming an undercut scroll thread in an uppermostsurface of the body portion before applying the composite layer;disposing the body portion in a mold; mixing the thermoset resin, theinsulating component, the strengthening fibers, and the impacttoughening additive; disposing the mixture on the uppermost surface ofthe body portion in the mold; molding the mixture to the uppermostsurface to form the composite layer; the molding step including applyingpressure to the mixture on the body portion of up to p to 2.5tons/square inch, and heating the mixture to 120 to 130° C. for 5 to 60minutes such that the mixture does not fully cure; and the molding stepincluding increasing the temperature of the compressed mixture to 180 to250° C. while applying pressure to the mixture on the body portion untilthe mixture fully cures and forms the composite layer.
 9. A method ofmanufacturing a piston, comprising the steps of: applying a compositelayer to a body portion formed of metal, the composite layer forming atleast a portion of a combustion surface, and the composite layerincluding a thermoset resin, an insulating component, strengtheningfibers, and an impact toughening additive, wherein the thermoset resinis present in an amount of 25 wt. % to 35 wt. %, based on the totalweight of the composite layer; the insulating component is present in anamount of 50 wt. % to 70 wt. %, based on the total weight of thecomposite layer; the strengthening fibers are present in an amount of 1wt. 00 to 10 wt. %, based on the total weight of the composite layer;and the impact toughening additive is present in an amount of I wt. % to10 wt. %, based on the total weight of the composite layer.